In recent years, color liquid crystal display devices have been used in various fields, such as notebook personal computers, desktop personal computers, tablet terminals, smartphones, portable telephones, PDAs, car navigation devices, PNDs, game devices, and portable music players. The color liquid crystal display device is provided with liquid crystal cells and a backlight. Example backlight structures include a direct structure where the light source is disposed immediately under the liquid crystal cells via a diffuser plate, and an edge light-type structure where the light source is disposed on the side of a light guide plate.
In such backlights, there are provided optical members such as a light diffusion sheet layered on the light-emitting surface of the light guide plate or diffuser plate for making the light from the light source uniform and a prism sheet for increasing the front side brightness (Patent Document 1).
Particularly in recent years, in the fields of tablet terminals, smartphones, portable telephones and the like, an environment enabling the viewing of higher definition pictures is available. In order to enable the viewing of the high-definition pictures, the liquid crystal pixels of the liquid crystal cells provided in the liquid crystal display device are becoming increasingly smaller. When such liquid crystal pixels are used, the transmittance of the light from the light source is markedly decreased compared with conventional art, resulting in poor front side brightness.
In order to overcome the decrease in front side brightness, improvements in optical members have been made (Patent Document 2). For example, in Patent Document 2, the prism sheet is designed with an increased refractive index compared with conventional art so as to increase the front side brightness.    Patent Document 1: JP-A-9-127314 (Claim 1, paragraph 0034)    Patent Document 2: JP-T-2008-503774
Patent Document 2 discloses a prism sheet having high refractive index (which may hereafter be referred to as “high refractive index prism sheet”) and a display device using the same. However, the literature does not provide any concrete description as to the light diffusion sheet to be combined with the high refractive index prism sheet. The high refractive index prism sheet has emitting angle characteristics concerning the emission of the light incident on the prism sheet which are different from those of conventional prism sheets. Thus, if the conventional light diffusion sheet is simply used, the emitting angle characteristics of the high refractive index prism sheet cannot be sufficiently effectively utilized. As a result, the backlight device cannot provide sufficient front side brightness and diffusing property.